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diff --git a/Documentation/sound/designs/timestamping.rst b/Documentation/sound/designs/timestamping.rst new file mode 100644 index 000000000..7c7ecf5db --- /dev/null +++ b/Documentation/sound/designs/timestamping.rst @@ -0,0 +1,215 @@ +===================== +ALSA PCM Timestamping +===================== + +The ALSA API can provide two different system timestamps: + +- Trigger_tstamp is the system time snapshot taken when the .trigger + callback is invoked. This snapshot is taken by the ALSA core in the + general case, but specific hardware may have synchronization + capabilities or conversely may only be able to provide a correct + estimate with a delay. In the latter two cases, the low-level driver + is responsible for updating the trigger_tstamp at the most appropriate + and precise moment. Applications should not rely solely on the first + trigger_tstamp but update their internal calculations if the driver + provides a refined estimate with a delay. + +- tstamp is the current system timestamp updated during the last + event or application query. + The difference (tstamp - trigger_tstamp) defines the elapsed time. + +The ALSA API provides two basic pieces of information, avail +and delay, which combined with the trigger and current system +timestamps allow for applications to keep track of the 'fullness' of +the ring buffer and the amount of queued samples. + +The use of these different pointers and time information depends on +the application needs: + +- ``avail`` reports how much can be written in the ring buffer +- ``delay`` reports the time it will take to hear a new sample after all + queued samples have been played out. + +When timestamps are enabled, the avail/delay information is reported +along with a snapshot of system time. Applications can select from +``CLOCK_REALTIME`` (NTP corrections including going backwards), +``CLOCK_MONOTONIC`` (NTP corrections but never going backwards), +``CLOCK_MONOTIC_RAW`` (without NTP corrections) and change the mode +dynamically with sw_params + + +The ALSA API also provide an audio_tstamp which reflects the passage +of time as measured by different components of audio hardware. In +ascii-art, this could be represented as follows (for the playback +case): +:: + + --------------------------------------------------------------> time + ^ ^ ^ ^ ^ + | | | | | + analog link dma app FullBuffer + time time time time time + | | | | | + |< codec delay >|<--hw delay-->|<queued samples>|<---avail->| + |<----------------- delay---------------------->| | + |<----ring buffer length---->| + + +The analog time is taken at the last stage of the playback, as close +as possible to the actual transducer + +The link time is taken at the output of the SoC/chipset as the samples +are pushed on a link. The link time can be directly measured if +supported in hardware by sample counters or wallclocks (e.g. with +HDAudio 24MHz or PTP clock for networked solutions) or indirectly +estimated (e.g. with the frame counter in USB). + +The DMA time is measured using counters - typically the least reliable +of all measurements due to the bursty nature of DMA transfers. + +The app time corresponds to the time tracked by an application after +writing in the ring buffer. + +The application can query the hardware capabilities, define which +audio time it wants reported by selecting the relevant settings in +audio_tstamp_config fields, thus get an estimate of the timestamp +accuracy. It can also request the delay-to-analog be included in the +measurement. Direct access to the link time is very interesting on +platforms that provide an embedded DSP; measuring directly the link +time with dedicated hardware, possibly synchronized with system time, +removes the need to keep track of internal DSP processing times and +latency. + +In case the application requests an audio tstamp that is not supported +in hardware/low-level driver, the type is overridden as DEFAULT and the +timestamp will report the DMA time based on the hw_pointer value. + +For backwards compatibility with previous implementations that did not +provide timestamp selection, with a zero-valued COMPAT timestamp type +the results will default to the HDAudio wall clock for playback +streams and to the DMA time (hw_ptr) in all other cases. + +The audio timestamp accuracy can be returned to user-space, so that +appropriate decisions are made: + +- for dma time (default), the granularity of the transfers can be + inferred from the steps between updates and in turn provide + information on how much the application pointer can be rewound + safely. + +- the link time can be used to track long-term drifts between audio + and system time using the (tstamp-trigger_tstamp)/audio_tstamp + ratio, the precision helps define how much smoothing/low-pass + filtering is required. The link time can be either reset on startup + or reported as is (the latter being useful to compare progress of + different streams - but may require the wallclock to be always + running and not wrap-around during idle periods). If supported in + hardware, the absolute link time could also be used to define a + precise start time (patches WIP) + +- including the delay in the audio timestamp may + counter-intuitively not increase the precision of timestamps, e.g. if a + codec includes variable-latency DSP processing or a chain of + hardware components the delay is typically not known with precision. + +The accuracy is reported in nanosecond units (using an unsigned 32-bit +word), which gives a max precision of 4.29s, more than enough for +audio applications... + +Due to the varied nature of timestamping needs, even for a single +application, the audio_tstamp_config can be changed dynamically. In +the ``STATUS`` ioctl, the parameters are read-only and do not allow for +any application selection. To work around this limitation without +impacting legacy applications, a new ``STATUS_EXT`` ioctl is introduced +with read/write parameters. ALSA-lib will be modified to make use of +``STATUS_EXT`` and effectively deprecate ``STATUS``. + +The ALSA API only allows for a single audio timestamp to be reported +at a time. This is a conscious design decision, reading the audio +timestamps from hardware registers or from IPC takes time, the more +timestamps are read the more imprecise the combined measurements +are. To avoid any interpretation issues, a single (system, audio) +timestamp is reported. Applications that need different timestamps +will be required to issue multiple queries and perform an +interpolation of the results + +In some hardware-specific configuration, the system timestamp is +latched by a low-level audio subsystem, and the information provided +back to the driver. Due to potential delays in the communication with +the hardware, there is a risk of misalignment with the avail and delay +information. To make sure applications are not confused, a +driver_timestamp field is added in the snd_pcm_status structure; this +timestamp shows when the information is put together by the driver +before returning from the ``STATUS`` and ``STATUS_EXT`` ioctl. in most cases +this driver_timestamp will be identical to the regular system tstamp. + +Examples of timestamping with HDAudio: + +1. DMA timestamp, no compensation for DMA+analog delay +:: + + $ ./audio_time -p --ts_type=1 + playback: systime: 341121338 nsec, audio time 342000000 nsec, systime delta -878662 + playback: systime: 426236663 nsec, audio time 427187500 nsec, systime delta -950837 + playback: systime: 597080580 nsec, audio time 598000000 nsec, systime delta -919420 + playback: systime: 682059782 nsec, audio time 683020833 nsec, systime delta -961051 + playback: systime: 852896415 nsec, audio time 853854166 nsec, systime delta -957751 + playback: systime: 937903344 nsec, audio time 938854166 nsec, systime delta -950822 + +2. DMA timestamp, compensation for DMA+analog delay +:: + + $ ./audio_time -p --ts_type=1 -d + playback: systime: 341053347 nsec, audio time 341062500 nsec, systime delta -9153 + playback: systime: 426072447 nsec, audio time 426062500 nsec, systime delta 9947 + playback: systime: 596899518 nsec, audio time 596895833 nsec, systime delta 3685 + playback: systime: 681915317 nsec, audio time 681916666 nsec, systime delta -1349 + playback: systime: 852741306 nsec, audio time 852750000 nsec, systime delta -8694 + +3. link timestamp, compensation for DMA+analog delay +:: + + $ ./audio_time -p --ts_type=2 -d + playback: systime: 341060004 nsec, audio time 341062791 nsec, systime delta -2787 + playback: systime: 426242074 nsec, audio time 426244875 nsec, systime delta -2801 + playback: systime: 597080992 nsec, audio time 597084583 nsec, systime delta -3591 + playback: systime: 682084512 nsec, audio time 682088291 nsec, systime delta -3779 + playback: systime: 852936229 nsec, audio time 852940916 nsec, systime delta -4687 + playback: systime: 938107562 nsec, audio time 938112708 nsec, systime delta -5146 + +Example 1 shows that the timestamp at the DMA level is close to 1ms +ahead of the actual playback time (as a side time this sort of +measurement can help define rewind safeguards). Compensating for the +DMA-link delay in example 2 helps remove the hardware buffering but +the information is still very jittery, with up to one sample of +error. In example 3 where the timestamps are measured with the link +wallclock, the timestamps show a monotonic behavior and a lower +dispersion. + +Example 3 and 4 are with USB audio class. Example 3 shows a high +offset between audio time and system time due to buffering. Example 4 +shows how compensating for the delay exposes a 1ms accuracy (due to +the use of the frame counter by the driver) + +Example 3: DMA timestamp, no compensation for delay, delta of ~5ms +:: + + $ ./audio_time -p -Dhw:1 -t1 + playback: systime: 120174019 nsec, audio time 125000000 nsec, systime delta -4825981 + playback: systime: 245041136 nsec, audio time 250000000 nsec, systime delta -4958864 + playback: systime: 370106088 nsec, audio time 375000000 nsec, systime delta -4893912 + playback: systime: 495040065 nsec, audio time 500000000 nsec, systime delta -4959935 + playback: systime: 620038179 nsec, audio time 625000000 nsec, systime delta -4961821 + playback: systime: 745087741 nsec, audio time 750000000 nsec, systime delta -4912259 + playback: systime: 870037336 nsec, audio time 875000000 nsec, systime delta -4962664 + +Example 4: DMA timestamp, compensation for delay, delay of ~1ms +:: + + $ ./audio_time -p -Dhw:1 -t1 -d + playback: systime: 120190520 nsec, audio time 120000000 nsec, systime delta 190520 + playback: systime: 245036740 nsec, audio time 244000000 nsec, systime delta 1036740 + playback: systime: 370034081 nsec, audio time 369000000 nsec, systime delta 1034081 + playback: systime: 495159907 nsec, audio time 494000000 nsec, systime delta 1159907 + playback: systime: 620098824 nsec, audio time 619000000 nsec, systime delta 1098824 + playback: systime: 745031847 nsec, audio time 744000000 nsec, systime delta 1031847 |